I didn't get a single response to my previous message, so I am reposting. Mail Server has a bug.
If username@yourbbsdomain.com tries to email moe@xxxyourbbsdomain.com, where xxx is any alphanumeric character(s), mail server attempts to deliver that mail to moe@yourbbsdomain.com if there exists a moe in User Manager, which is wrong. If moe isn't in User Manager, the email will be rejected by Mail Server without ever getting off the local server (i.e., Mail Server's SMTP client doesn't even try to deliver to your ISP's SMTP Server to find xxxyourbbsdomain.com).
As proof, while on spiderisland, I mailed Michael_Davidson@aspiderisland.com (notice the "a" in front of spiderisland), and the mail was delivered to my email account, which is wrong. Try it on your own system.